Global Warming Articles:
Reading the Tree-Rings
Massive Pinyon Pine Die-off
Polar Melting
Hard Numbers on Global Warming
African Lake Demonstrates Effects of Global Warming
Global Climate Change Lecture Series
Fall 2006 Alumnus Global Warming update:
Forest Fires
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The University of Arizona Alumnus — Summer 2006 / Global Warming Reading the Tree-Rings
Some consider global warming jittery, left-wing conjecture. Others say planetary end times are nigh. Then there are those who stick to the big picture — and use a big stick to do so. People like Malcolm Hughes, who at the moment is wrestling a cumbersome chunk of giant sequoia from the corner of his squat office, inside the concrete bowels of Arizona Stadium.
Minutes after my arrival, Hughes, a jocular and noted dendrochronologist or tree-ringer researcher, is nearly beaning me with his sequoia remnant. Ah, the sacrifices we make for science. Still, there’s a dead-serious purpose behind this madness. As part of The University of Arizona’s famed Laboratory of Tree-ring Research, Professor of Dendrochronology Hughes has spent decades reading those rings for clues to past climates. Now he’s using them to divine the future. And what he sees isn’t comforting. “It seems very likely,” Hughes says, “that we’re having unusual, rapid, and synchronous warming across many regions of the northern hemisphere. It’s markedly more synchronous than any other relatively warm periods 600, 800 or 1,000 years ago.” That’s a fancy way of saying the planet is getting hotter, in different and more troubling ways than ever before. Hughes’ deduction is garnered from tree rings where thickness — one ring per year — reflects climate patterns. And like a snapshot, each of those rings records the atmosphere’s chemical composition for any given point in time. Not surprisingly, such hypotheses stir passions on both sides of the global warming fence. Believers claim this research as yet another nail in the skeptics’ coffin. Others say Hughes’ projections — and those of his colleagues — are flawed, or at least misleading. One controversy arose over a 1998 article, appearing in the journal Nature that Hughes coauthored with Ray Bradley of the University of Massachusetts, and the University of Virginia’s Michael Mann. In their piece, the scientists presented the tendentious “hockey stick” model for global change: planetary temperatures had remained steady for a period of time — like a hockey stick shaft, they argued — before shooting upwards after 1900. Ergo, the blade. This iconic image struck home. It also outraged global warming naysayers, and even other scientists who disagreed with the model’s particular temperature timeline. Anger reached fever pitch when the hockey stick was tapped for a cautionary U.N. report on greenhouse gasses. Last year, Hughes was the center of another squabble, this time in the U.S. House of Representatives, over the premise of global warming itself. The ruckus arose when Texas Republican Joe Barton, chair of the Energy and Commerce Committee, sparked a probe into a several scientists — including Hughes — who were charting global warming with the assistance of federal funds. Barton has been an outspoken critic of global warming theories, and vigorously opposes attempts to limit greenhouse gas-causing emissions. But his efforts to stifle these leading scientists drew a sharp response from fellow congressmen such as New York Republican Sherwood Boehlert, chair of the House Science Committee. “My primary concern about your investigation,” Boehlert wrote in a letter to Barton, “is that its purpose seems to be to intimidate scientists rather than to learn from them, and to substitute congressional political review for scientific review.” The lesson? That science and politics are never far apart, and there should be little surprise that controversy might touch a tree-ring researcher with his fingers on the climatological pulse of several millennia. If Hughes is to blame for stirring the scientific stew, however, he’s in good company; the UA is known for its tradition of vigorous scientific debate — a custom now fully engaged with discussions over global change. That ongoing parley ranges from documentation of drought-related pinyon pine die-offs by the School of Natural Resources, or Geosciences department research on plunging fish yields in Africa’s Lake Tanganyika — believed caused by climate shifts — to charting of rapid climate changes in the Antarctic by John Overpeck, director of the UA’s Institute for the Study of Planet Earth. In fact, that institute — called ISPE for short — has become a central stomping grounds for climatological research conducted across campus. “Our goal is promoting interdisciplinary research on global change, and certainly global warming is a big part of that,” says Barbara Morehouse, ISPE’s deputy director. “We encourage people in different departments to get together in a collaborative way, and we try to build bridges between science and society.” Founded in 1994, with the goal of fostering disciplinary excellence, ISPE appears to have done just that, attracting scientists like Malcolm Hughes to join its executive committee. With the help of ISPE, Hughes can meld his research to many other studies being carried out at the UA. The result is a ripple effect that’s both broad and narrowly precise. At least that’s the case with tree rings’ ability to pinpoint exact moments in the planet’s history, and then repeat those portraits at various spots across the globe. “It’s just like the mayor of a city burying a time capsule,” says Hughes. “Now, you could dig that mayor’s time capsule up. But instead of just digging up one, we could dig up a whole series of them, covering thousands of years.” Hughes says his work fits within a larger academic movement called earth systems science. This perspective was sparked, in part, by images of the earth from Apollo space missions. “That suddenly showed us a new way of looking at the planet, even as a whole new set of tools was becoming available — computation, remote sensing, techniques for knowing exactly where on the earth’s surface something was happening. Techniques, ideas, imagination — they were all starting to come together.” And tree rings? They were at the very center of that cosmic shift. “Tree rings are a library of the past,” Hughes explains. “Each year, the trees of a large part of the world form a new growth ring. The size and structure of that ring is controlled in part by what’s going on in the (surrounding) environment. “In the simplest sense, if a tree is growing near waterfalls, the rings will be big and fat. But if you were to move up the slope a little bit, you see that trees rings are limited.” “The beauty,” says Hughes, “is that tree rings re-create this long series of years — from dozens to thousands — laid out in front of us in the right sequence. And we are able to give them calendar dates.” That means scientists can pinpoint not only droughts and rainy periods, but also record elements in the atmosphere, year by year, endlessly. “So you’ve got your perfect little time capsule,” Hughes says. “They’re in sequence, one for every year — the carbon, hydrogen, and oxygen in the environment for that year.” Take the sequoia plank. Cut in the 1880s, and damaged by fire before that, its remaining, outermost rings date to A.D. 1562. “They reveal patterns of broad and narrow rings,” he says, “that we see on all giant sequoia in a 160-to-170-mile, north-south (stretch) on the western side of the Sierra Mountains.” For example, he can trace a 16th century drought (“Almost all the rings in the 1580s were small rings”), and another in 1977. “The link is straightforward for patterns of behavior in weather and climate,” Hughes says. “That leads us to two main methods for looking at very large-scale change. Both involve getting extensive geographical coverage back to about 1600.” But even as that big picture is pieced together, several factors affecting past climates are eliminated. For example, “the continents haven’t moved since 1600,” he says. “Also, the alignment of the Earth and sun followed normal patterns.” In other words, “we’ve got an Earth pretty much the same during industrial and preindustrial times.” That makes it easier to isolate what may be causing the changes. “We can then use the tree rings,” he says, “to see if the variability is about the same or what is characteristically different.” And what do those records show? “When you take the estimate of regional temperatures from tree rings and other records over the last 600 years, they look very much like what you get from computer models.” In turn, those models “are being driven by our best estimates of input from the sun, and the effect of the bigger volcanoes, until you get into the late 19th or 20th century.” And that’s when the hockey stick takes a sharp, upwards turn. “You can only match those observations — and the instrument records — by invoking human-caused greenhouse warming,” Hughes says. He stands, gingerly returning that sequoia plank to its corner. And frankly, compared to the hockey stick, that ancient tree is looking better all the time.
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